Evaluation of the Effect of Training on the Anterior Belly of the Digastric Muscle Using Ultrasound Elastography.

Background: Neuromuscular electrical stimulation (NMES) strengthens muscles and is used to treat dysphagia. Assessing NMES training at rest is preferable because patients may experience difficulty following instructions.

Objectives: To determine whether muscle hardness, circumference and area changes can be measured using ultrasound real-time tissue elastography (RTE) after NMES of the anterior belly of the digastric muscle and whether these measurements can be an index for muscle activity during NMES.

Methods: Overall, 21 healthy adults were included in this study. Baseline hardness, circumference and area of the anterior belly of the digastric muscle at rest and with the mouth open, as well as the muscle activity of the submandibular surface layer when the mouth was open, were measured using RTE and surface electromyography. NMES was performed for 30 min weekly for 4 weeks, and similar measurements were taken at 2 and 4 weeks after the start of stimulation.

Results: Muscle hardness at rest and with the mouth open differed significantly between baseline and Week 4 (resting: p = 0.0048; mouth open: p = 0.034). The strain ratio at Weeks 2 and 4 correlated negatively with the maximum voluntary contraction (Week 2: r = -0.77, p < 0.01; Week 4: r = -0.564, p < 0.05). A positive correlation was found between muscle hardness at rest and with the mouth open at Week 4 (r = 0.544, p < 0.05).

Conclusion: Our results indicate that muscle hardness at rest determined using RTE may be utilised to assess the influence of NMES. This measurement could serve as an objective indicator for implementing NMES.